Python __isub__() Magic Method

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Syntax

object.__isub__(self, other)

The Python __isub__() magic method implements in-place subtraction x -= y that subtracts the operands from each other and assigns the result to the left operand. This operation is also called augmented arithmetic assignment. The method simply returns the new value to be assigned to the first operand.

  • When you call x -= y, Python first attempts to call x.__isub__(y).
  • If this is not implemented, it tries the normal subtraction x.__sub__(y).
  • If this is not implemented either, it tries reverse subtraction y.__rsub__(x) with swapped operands.

The result is then assigned to the first operand x. If none of those operations is implemented, Python raises a TypeError.

We call this a “Dunder Method” for Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Basic Example Overriding __isub__

In the following code example, you create a class Data and define the magic method __isub__(self, other).

  • The “self” argument is the default argument of each method and it refers to the object on which it is called—in our case, the first operand of the in-place operation.
  • The “other” argument of the in-place method refers to the second operand, i.e., y in the in-place operation x -= y.

The return value of the operation returns a dummy string 'finxter 42' to be assigned to the first operand. In practice, this would be the result of the in-place subtraction.

class Data:
    def __isub__(self, other):
        return 'finxter 42'


x = Data()
y = Data()

x -= y

print(x)
# finxter 42

In-Place Subtraction Without __isub__()

To support in-place subtraction on a custom class, you don’t have to overwrite the in-place __isub__() method. Because if the method is not defined, Python will fall back to the normal __sub__() method and assign its result to the first operand.

Here’s an example:

class Data:
    def __sub__(self, other):
        return 'finxter 42'


x = Data()
y = Data()

x -= y

print(x)
# finxter 42

In-Place Subtraction Without __isub__() and __sub__()

To support in-place subtraction x -= y on a custom class, you don’t even have to overwrite any of the x.__isub__(y) or x.__sub__(y) methods. If both are not defined, Python falls back to the reverse y.__rsub__(x) method and assigns its result to the first operand.

Here’s an example where you create a custom class for the first operand that doesn’t support the subtraction operation. Then you define a custom class for the second operand that defines the __rsub__() method. For the in-place operation, Python falls back to the __rsub__() method defined on the second operand and assigns it to the first operand x:

class Data_1:
    pass

class Data_2:
    def __rsub__(self, other):
        return 'finxter 42'

x = Data_1()
y = Data_2()

x -= y

print(x)
# finxter 42

TypeError: unsupported operand type(s) for -=

If you try to perform in-place subtraction x -= y but neither x.__isub__(y), nor x.__sub__(y), nor y.__rsub(x) is defined, Python raises a “TypeError: unsupported operand type(s) for -=“. To fix this error, simply define any of those methods before performing the in-place operation.

class Data:
    pass


x = Data()
y = Data()

x -= y

Output:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 8, in <module>
    x -= y
TypeError: unsupported operand type(s) for -=: 'Data' and 'Data'

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References:

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